Question

Define
1 allergy
2 allergen
With example

Answer 1

1) Allergy is a damaging hypersensitive immune response by the body to a substance, like a particular food, pollen, fur, dust, etc.

If you need a simpler definition,

Allergy is an abnormal reaction of the immune system of our body to a foreign substance.

Allergies are exaggerated-it means, we have a big response from our body like swelling, itching etc .

Examples: Food allergy, Asthma, redness, swelling, etc.

Allergens: Any substance that triggers an allergic response from the immune system is called an allergen.

A simpler definition is:
Allergens are substances that causes allergies.

Examples: dust, pollen, some medicines etc.